Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MAREK'S DISEASE VIRUS VACCINE
The present invention relates to a new strain of Marek's Disease Virus (MDV), and to
a vaccine for the protection of poultry against Marek's Disease (MD) cont~ining this novel
strain. The invention is also concerned with a process for the prepa. alion of such a vaccine.
Marek's Disease is a malignant, Iympho-proliferative disease of domestic fowl, caused
by an infection with a herpesvirus: Marek's Disease Virus. MD is ubiquitous, occurring in
o poultry-producing countries throughout the world. The causative virus is highly contagious
and readily spreads to susceptible birds. Chickens raised under intensive production systems
will inevitably suffer losses from MD.
MD affects chickens from about 6 weeks of age, occurring most frequently between the ages
of 12 and 24 weeks.
Three forms of MD are recognized clinically, classical MD, acute MD and transient
paralysis.
Classical MD is characterized by peripheral nerve enlargement caused by Iymphoidinfiltration and demyelination, and paralysis is the dominant clinical sign. Mortality is variable,
but normally under 10 to 15 %.
In the acute form there are multiple and diffuse Iymphomatous tumours in the visceral
organs. Mortality from this form of MD is usually higher than from the classical form. An
incidence of 10 to 30% is common in unvaccinated flocks and outbreaks involving up to 70%
of the flock may occur. The pathological lesions in both classical and acute MD are essçnti~lly
the same, involving the proliferation and infiltration of malignantly transformed T-
lymphoblasts into normal tissues, peripheral nerves in the case of the classical form, and
visceral organs in the case of the acute form.
Furthermore, MDV has been shown to be responsible for encephalitis in young
chickens, which is characterized by sudden paralysis.
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There are three distinct serotypes of MD related viruses found in chickens:
TypeI : the pathogenic and oncogenic form responsible for the disease in chickens,
including high and low virulence forms and ~ttçnll~ted non-pathogenic strains
derived therefrom;
5 Type II : non-pathogenic and non-oncogenic strains of MDV;
Type III : herpesvirus of turkeys (HVT), which is non-pathogenic to chickens.
Several practical Marek's Disease vaccines have been developed and are currently in
use today. One of the earliest MD vaccines consisted of the serotype III virus, which was
originally isolated from turkeys, (see Witter et al. Am. J. Vet. Res., 31, 525-538, (1970)).
HVT is used extensively as a vaccine against M:D. It is commonly used as a cell-associated
prepal~lion, however, substantial amounts of cell-free virus can be extracted from infected
cells and a cell-free vaccine has been described in US patent No. 3,647,861.
Serotype II MD viruses are naturally occurring non-oncogenic viruses, which thus do
not have the potential for causing tumours in vaccinated chickens. These viruses do not, in
consequence, require any artificial attenuation by serial p~.cs~ging and, since they are in their
natural state, cannot revert to a virulent form. The SB-1 strain (US patent No. 4,160,024) was
originally administered as a cell-associated preparation. Such a vaccine, in practice, has to be
stored and transported in liquid nitrogen at about -196C. This serotype II strain has been
found to be poorly protective on its own and is, therefore, usually admini.~tered in combination
with HVT as a bivalent vaccine, since the two viruses together produce greater protection
than does either one alone. This phenomenon is called "protective synergy".
2s European patent application No. 90 314297 describes a vaccine comprising a cell-free
form of SB-l . Even in the cell-free form the SB- 1 strain is still not very protective on its own
and is, in consequence, administered with HVT.
Several vaccines comprising attenuated serotype I MDV have been developed and are
in use today.
WO 85/04588, for example, describes an attenuated strain derived from the parent virus MDV
CVl-988. All the vaccine preparations described in this publication contained the virus in cell-
associated form.
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US patent application No. 7 723 037 describes an attenuated revertant serotype I MD
vaccine. The strain used in this vaccine does not produce cell-free virus.
s Thus, to date all serotype I vaccines have had to be ~mini~tered as cell-associated
preparations, with the attendant disadvantages of having to be stored and transported in liquid
nitrogen. When a vaccine is not stored or handled correctly th~ere is a decrease in the viability
of the virus with consequential failure of the vaccination. In countries where liquid nitrogen
storage is not practical or available, it is impossible to employ cell-associated MD vaccine.
Furthermore, the MDV cont~ining particles suspended in a cell-associated prepal~lion
are liable to vaccine precipitate, and therefore the suspension requires homogenization before
~timini~tration. Inadequate homogenization may result in an incorrect dose of vaccine and
consequently lead to failure of the vaccination. Moreover, the strictly cell-associated nature of
s said vaccines is responsible for the susceptibility of the vaccines to physical abuse. Damage to
the infected cells by sub-optimal harvesting and freezing procedures, as well as incorrect
thawing of the ampoules and handling of the vaccine at the hatcheries, will cause cell damage
and death and subsequent loss of vaccine titres.
Maternal derived antibodies (MDA) to all MD viral serotypes are ubiquitous in
commercial chicks, due to natural exposure of breeders to MD viruses and/or vaccination of
breeders with serotypes I, II and III viruses. Such MDA are passed to the offspring and reduce
the efficacy of any subsequent vaccination.
Whilst it is impossible to control MDA to serotypes I and II MDV, it is possible to control
MDA to serotype III MDV, i.e. HVT, by vaccin~ting breeder flocks with MDV vaccines
lacking HVT, so that their progeny may be better protected when vaccinated with HVT-
cont~ining monovalent, bivalent or polyvalent vaccines. Such a vaccination strategy is called
"alternate generation" vaccination. In countries where liquid nitrigen storage and transporting
facilities are not available it is not possible to practice this alternate generation vaccination,
since the only freeze-dried MDV vaccine available contains HVT. Thus there is a need for a
freeze-dried serotype I MDV vaccine.
Earlier work with serotype I MD viruses demonstrated that the amount of cell-free
virus, measured as pfu (plaque forming units) was of inadequate titre to be useful for
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vaccination purposes (US patent 4,895,718; Witter, R.L. et al., Avian Diseases 31,829, 1987;
Powell, P.C., World's Poultry Science Journal 42, 205, 1986; Schat, K.A., Internews 3, 13,
1989).
s However, a new strain of serotype I MDV has been found which, following
~ttçn--ation by serial passage in chick cell cultures produces large amounts of cell-free virus
and is more protective than any serotype I MDV used to date. As a result it is possible to
produce a freeze-dried serotype I MDV vaccine. Even when cell-associated this strain has
been found to be more protective than the best MDV serotype I vaccine currently available.
According to one aspect of the invention there is provided a novel ~tt~n-~ated strain of
Marek's Disease Virus serotype I which, when liberated from infected cells by sonication,
results in, at most, a 100-fold decrease in cell-free virus titre compared with the cell-associated
virus titre.
Preferably the attenuated strain produces at most a 50-fold decrease in cell-free virus
titre, most preferably a 10-fold decrease in cell-free virus titre. Clearly it is desirable that,
following sonication, there is no drop in cell-free virus titre, when compared with the cell-
associated virus titre.
An ~ttenu~ted cell-free strain of MDV serotype I in accordance with the present
invention was deposited on 24 June 1994 with the European Collection of Animal Cell
Cultures, Porton Down, United Kingdom under the Budapest Treaty, and designated
accession No. V94062211.
One strain of MDV serotype I, in accordance with the invention and hereinafter
referred to as MR22, was isolated on chick embryo kidney cell cultures from buffy coat cells
taken from a flock in the field in 1971. It was then passaged 18 times in chick embryo
fibroblast (CEF) cell cultures prepared from SPF (specific pathogen free) eggs. This isolate
was passaged a further two times in secondary CEF cells, then a cell free prepalalion was
made by sonicating infected cells in the presence of S.P.G.A. stabilizer (Bovarnik et al., J.
Bact., 59, 509. 1950.).
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The MR22 strain obtained as described above was shown to be an MDV serotype I
virus by the fact that it reacted with monoclonal antibodies 2092 and 4859, but failed to react
with monoclonal antibodies that react with serotype II and III ~irus. These antibodies were
prepared and used as described by Lee, L.F., J. Immunology, 130, 1003-1006, (1983), and
s Silva and Lee, Virology, 136, 307-320, (1984). Table 1 shows the results of fluorescent
antibody studies using dirrerenl MDV strains/serotypes against specific monoclonal antibodies.
Table 1
The reaction pattern of M.D. strains with four monoclonal antibodies (fluorescent antibody
0 results).
Virus Sero-Monoclonal Monoclonal Monoclonal Monoclonal
type H19 2BN Y5.9 L78.2
HPRS 16 1 ++ ++
VICTORIA 1 + +
RISMAVAC 1 - +
CLONE C 1 - +
MR22 1 + +
LPSB 1 2 - - ++
HP SB1 2 - - ++ (+/-)
HVT/PB 1 3 - - - ++
Table 2 shows that the amount of cell free MR22 virus obtained when infected chick
embryo fibroblasts are sonicated in the presence of the stabilizer SPGA is considerably greater
than other serotype I strains available.
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Table 2
Strain SerotypeCell associated Cellfree Difference
titre (pfu/ml) titre (pfu/ml)
HPRS-16 1 106 ~1o2 >10 000
Rispens (CVI988) 1 106 1o2 10 000
MR22 1 1057 104.7 10: 1
The MR22 strain was further passaged through a chicken, virus was isolated
subsequently and passaged twice in CEF after which the virus was passaged in a chicken as a
cell-free virus. After re-isolation the virus was passaged a further six times in CEF (=MSV,
master seed virus). The potency of the virus to release high amounts of cell-free virus was
tested several times in separate experiments.
Table 3
Passage level Titre of virus yield (pfu/ml)Difference
Cell associated Cell free
MSV+2 1o6.3 1o4.8 32: 1
MSV+6 1o6.9 105.4 32:1
MSV + 6 1o6.8 106a 5
MSV+6 1o5.6 1047 8: 1
A further MDV serotype 1 strain which produces a large amount of cell free virus is
Victoria 10, a strain isolated from a healthy flock in the UK in the mid-eighties. The virus was
passaged 10 times in chicken kidney cells (CK) and 18 times in CEFs. The development of cell
free virus yield is given below:
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Table 4
Passage levelTitre of virus yield (pfu/ml) Difference
Cell associated Cell free
CKlOCEF6 1059 103.4 316: 1
CKlOCEF11 1o6.2 1oS.2 10
CKlOCEF18 1o5.6 1o4.6 10 ~ 1
s The advantage of this greater production of cell- free virus by MR22, Victoria 10 or
any other MDV serotype 1 strain is that it is possible to prepare a freeze-dried vaccine
preparation, which in turn removes the need for storing and transporting the vaccine in liquid
nitrogen. To date, no freeze-dried serotype I MDV vaccine has successfully been produced.
o To propagate the MR22 strain for vaccine production roller cultures seeded with CEF
cells can be inoculated with cell-associated or cell-free virus obtained as described above.
After an incubation period of several days the supernatant medium is discarded and the cells
removed with a trypsin-versene mixture, after which the cells are deposited by centrifugation
and the supernatent is discarded.
In order to prepare the cell-free virus the deposited cells are suspended in buffer, for
example in phosphate-buffered saline (PBS) or preferably in a medium cont~ining a stabilizer,
SPGA being the most preferred.
Cell disruption may be effected by several methods, e.g. sonication or freeze-thaw. The
presence of any intact cells can be determined by e~r~min~tion in a hemocytometer.
The sonicated or quick frozen preparation can be filled into vials and can then be freeze-dried,
if desired in the presence of EDTA. Optionally, before freeze-drying, the cellular debris is
removed by filtration or centrifugation.
2s
Cell-free serotype I MDV obtained by the method described above can be incorporated
into vaccines as live or inactivated virus.
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The vaccine cont~ining live virus can be prepared and marketed in the form of a
suspension, or it can be lyophilized.
s Lyophilized vaccines plerelably contain one or more stabilizers. Suitable stabilizers
include, for example, SPGA, carbohydrates such as sorbitol, mannitol, starch, dextran or
glucose, proteins such as albumin or casein, or degradation products thereof, and buffers such
as alkali metal phosphates. If desired, one or more compounds with adjuvant activity can also
be added. Suitable compounds for this purpose include vitamin E acetate o/w-emulsion,
o aluminium hydroxide, phosphate or oxide, mineral oil (such as Bayol F and Marcol 52
(registered trade marks)) and saponins.
As a matter of course, the MDV serotype 1 strains specifically mentioned herein, in
particular strain MR22 can be used as the active component in a MDV vaccine in a cell
associated form. Such vaccines, live or inactivated, can be prepared according to conventional
methods.
The aim of inactivation of the MD viruses is to elimin~te reproduction of the viruses.
In general, this can be achieved by chemical or physical means. Chemical inactivation can be
effected by treating the viruses with, for example, enzymes, formaldehyde, 13-propiolactone,
ethylene-imine or a derivative thereof,an organic solvent (such as Tween, Triton [Registered
Trade Marks], sodium desoxy-cholate, sulphobetain or cetyl trimethylammonium salts). If
necessary, the inactivating substance is neutralized afterwards; material inactivated with
formaldehyde can, for example, be neutralized with thiosulphate. Physical inactivation can
preferably be carried out by subjecting the viruses to energy-rich radiation, such as W light,
X-radiation or ~-radiation. If desired, the pH can be brought back to a value of about 7 after
treatment.
Usually, an adjuvant, selected from the list mentioned above, and, if desired, one or
more emulsifiers, such as Tween and Span (Registered Trade Marks), are also added to the
inactivated virus material.
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The vaccine is aAmini.ctered in an effective dosage of the viral agent, i.e. the amount of
immuni7ing cell-free virus material that will induce immlmity in a chicken against challenge by
a virulent ~ virus. Tmml-nity is defined as the induction of a significantly higher level of
protection in a population of chickens after vaccination, compared to an unvaccinated group.
For live vaccines the dose rate per chick may range from 1 to 6 logs10 pfu.
Typically, the live vaccine according to the invention is administered in a dose of at
least 2.2 logsl0 pfu cell-free virus, preferably in a dose of at least 2.7 logsl0 pfu cell-free virus,
0 more preferably in a dose of at least 3 2 logsl0 pfu.
In the case of a natural route of a~mini.ctration, e.g. spray,eye and nose drops, a dose
of 10 -10 pfu/chick may be a~mini~tered.
Inactivated vaccines may contain the antigenic equivalent of 3 to 7 logsl0 pfu per bird
dose, preferably between 4 to 6 logsl0 pfu.
Vaccines according to the invention may be administered by spray at high titre, eye
drop, nose drop, orally (e.g. in the drinking water), or by means of intramuscular,
20 subcutaneous or in ovo injection at any age after the chicken obtains immunocompetence.
Normally the vaccine is administered to the chick 24-48 hours after hatching.
Another aspect of this invention is the combination of cell-free MDV serotype I with
cell-free HVT as a bivalent vaccine. Surprisingly, it has been found that the cell-free MDV
25 serotype I is still able to augment the efficacy of HVT, despite the increased stage of
p~cS~gln~
In particular, cell-free serotype I MDV of the MR22 strain are used in combination
with cell-free HVT. The HVT virus to be incorporated into a vaccine according to the
30 invention may be of any available strain, e.g. FC126 or THV PBl (commercially available
from Intervet Inc.). Optionally, the HVT virus comprises a foreign gene encoding an antigen
of another poultry pathogen, inserted into its viral genome, forming a polyvalent vaccine.
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Still another aspect of this invention is the combination of cell-free MDV serotype I
with cell-free MDV serotype II as a bivalent vaccine, or with both cell-free MDV serotype II
and cell-free HVT as a trivalent vaccine. Preferably the SB-1 strain or the HPRS B-24 strain
are used as MDV serotype II strain. The MDV serotype II strain can also be genetically
5 manipulated to incorporate an antigen of another poultry pathogen.
The invention also includes combination vaccines comprising, in addition to the cell-
free serotype I MD viral material, a vaccine derived from other pathogens infectious to
poultry. The cell-free serotype I MDV can be administered in combination with a vaccine virus
o selected from the group consisting of Newcastle Disease virus (NDV), Infectious Bronchitis
virus (IBV) and Infectious Bursal Disease virus (IBDV).
Example 1
5 A. P~s~.in~ of Serotype l MD VirusMR22.
Following the initial isolation of MR22 on chick embryo kidney cell cultures it was
passaged on chick embryo fibroblast (CEF) cell cultures.
Cell associated MR22 virus is inoculated onto 24 hour old SPF derived CEF cell
cultures grown on 6cm dianeter Falcon Petridishes (1.5 x 10 CEF/dish).
0.1 ml of inoculum containing at least lOOpfu in inoculated into the 5ml of tissue
culture medium on the plates and the cell associated virus settles on the monolayer and infects
them.
After an incubation period of 5 days at 38.5 C in a CO atmosphere of 5%, the cells are
removed from the dishes by:-
1. Pouring offthe medium.
2. Adding trypsin versene PBS solution to loosen the attachment of the cells to the petri
dish.
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3. Discarding the trypsin/versene PBS mixture before the cells detach from the petri dish.
4. Washing the cells offthe dishes with growth medium.
The suspension of cell associated virus obtained from step 4 is used as inoculum for
the next passage on CEF cells. The viruses were passaged 18 times as described above.
B. Preparation of MR22 cell-free serutype 1 MD vaccine
Two roller cultures (1750 cm) seeded with 200 x 10 CEF cells were inoculated into
the medium with 1 ml of cell-associated MR22 seed virus, obtained by the method described
above, with a titre of approximately 10 pfu/ml after 24 hours incubation.
After a further incubation period of 5 days the supernatant medium was discarded and
the cells removed with a trypsin versene mixture. The cells were deposited by centrifugation,
the supernatent discarded and the cells mixed with 20 mls of SPGA stabilizer and then
ultrasonicated for 20 secs.
The sonicated preparation was filled out in 1 ml aliquots in vials and freeze dried.
Titre pre freeze drying 10 pfu/ml
Titre post freeze drying 10 pfu/ml.
Example 2
Con-par~ e efficacy of cell-free Mareks Disease Vaccines
Day-old SPF chicks were divided into groups of 30, placed in negative pressure
isolators and each group was vaccinated intramuscularly with 0.1 ml/chick of one of the
following vaccines or combination of vaccines:-
A) SBl (cell-free type II vaccine strain MR30, Intervet, Boxmeer, the Netherlands) at a
dose of 200 pfu/chick.
B) MR22 (cell-free type I vaccine) at a dose of 200 pfu/chick.
12 215~760
C) HVT (cell-free type III vaccine strain PB-1, Intervet) at a dose of 1000 pfu/chick and
SB1 (cell-free type II vaccine, strain MR30, Intervet) 200 pfu/chick.
D) HVT (cell-free type III vaccine strain PB-1, Intervet) at a dose of 1000 pfu/chick and
MR22 200 pfu/chick.
At 7 days post vaccination all groups together with another group of 30 SPF chicks
were challenged with virulent RBlB Mareks Disease virus at a dose rate of 250 pfu/chick,
0. lmVchick given intraml~sc~ rly
lo The chicks were observed until the end of the experiment at 91 days. Any chick that
died was autopsied and the cause of death established. At the end of the experiment all
re~ birds were killed and autopsied.
The number of chicks dying of MD by the end of the experiment indicated that thechallenge was severe (see Figure 1). All control chicks had died by 56 days; 90% due to
Mareks disease and 10% due to nonspecific causes. The birds rem~ining alive in the group
receiving SB1 vaccine alone were all killed at 63 days because they had started to show an
unacceptable level of MD, 13.3% having died from MD already by that time.
This experiment demonstrated that MR22 in a cell-free state given alone, or in
combination with HVT, protected well against a severe Mareks Disease challenge.
Example 3
Comparison of the immunity induced by MR22 + HVT vaccine compared with Rispens +HVT
The virus strains used were as follows:-
1. HVT cell-associated vaccine (strain FC 126-Intervet).
2. Rispens cell-associated vaccine (strain CVI988-Intervet).
3. MR22 cell-associated vaccine.
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Groups of approximately 40 one-day-old chicks were placed in negative pressure
isolators and vaccinated with one of the following colllbinalions of cell-associated vaccines.
The chicks were shown to have maternally derived antibodies to serotype I, II and III (~A-
positive).
Group A
These chicks received 1000 pfu of HVT and 1000 pfu of Rispens given in 0.1 ml
intr~ml-sc~ rly,
0 Group B
These chicks received 1000 pfu of HVT and 1000 pfu of MR22 vaccine both given in0.1 ml intr~mllsc~ rly.
At 5 days of age groups A and B were challenged together with a control group of 40
5-day-old unvaccinated chicks. All chicks received 500 pfu of the virulent Mareks virus RBlB
in the cell-associated form intr~mllsc~ rly. All birds were observed for 12 weeks. Any birds
showing signs of MD were killed and these, together with any bird that died, were autopsied
to establish the cause of death. Where necessary histological ex~min~tion of tissue was
performed. At the end of the experiment all birds were killed and autopsied to establish the
20 presence of MD lesions.
The incidence of MD in the control unvaccinated chicks was very high (see Table 5
and Figure 2). A high level of immunity was evident in both vaccinated groups with Group B
showing the better protection against this virulent challenge.
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14
Table 5
Incidence of M.D. following challenge with RB lB of control and vaccinated birds.
Number ofNon specific M.D. M.D. by Total%M.D.
birdsat startdeaths tumors histology M.D.
Controls 39 0 34 3 37 94
HVT+Rispens 41 0 9 4 13 31
HVT + MR22 38 1 3 2 5 13
These results demonstrate that, in the cell-associated form, MR22 + HVT protect
better against a severe challenge of RBlB MDV than Rispens + HVT, which is currently the
best vaccine available commercially.
